The present invention relates to telecommunications in general, and, more particularly, to a method and apparatus for de-assigning signals from the fingers of a rake receiver.
FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center (xe2x80x9cWSCxe2x80x9d) 120, which may be known also as a Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) or Mobile Telephone Switching Office (xe2x80x9cMTSOxe2x80x9d). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to local- and long-distance telephone offices (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal, which is connected to the system via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called xe2x80x9ccells.xe2x80x9d As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topography of the terrain serviced by the system. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
Typically, the signal transmitted by a wireless terminal to a base station is radiated omni-directionally from the wireless terminal. Although some of the signal that is transmitted radiates in the direction of the base station and reaches the base station in a direct path, if one exists, most of the transmitted signal radiates in a direction other than towards the base station and is never received by the base station. Often, however, signals that radiate initially in a direction other than towards the base station strike an object and are reflected towards, and are received by, the base station. Thus, a signal can radiate from the wireless terminal and be received by the base station via multiple signal paths.
FIG. 2 depicts a schematic illustration of wireless terminal 101-1 as it transmits to base station 103-1. Signal 107-1 is received by base station 103-1 directly. Signal 107-2, signal 107-3 and signal 107-4 arrive at base station 103-1 after radiating initially in a direction other than towards base station 103-1 and only after reflecting off of an object, such as buildings 105-2 through 105-4, respectively. Signals 108-1 through 108-4 radiate from wireless terminal 101-1 but never reach base station 103-1.
Because each of the four signals arrives at base station 103-1 after having traveled a different path, each of the four signals arrives phase-shifted with respect to each other. And furthermore, depending on the length of the path traveled and whether the signal is reflected off an object before reaching base station 103-1, the signal quality (e.g., the average power of an amplitude-modulated signal, the signal-to-noise ratio, absolute power in dBm, etc.) of each signal is different when received. This is partially due to the fact that when a signal is reflected off of an object, the degree to which the signal is attenuated is a function of, among other things, the angle at which the signal is incident to the object and the geometric and dielectric properties of the object.
FIG. 3a depicts an illustrative graph of the average power of an amplitude-modulated signal as a function of time for the direct path signal 107-1 in FIG. 2, which typically arrives at base station 103-1 in the best condition of all the constituent signals. FIG. 3b, 3c and 3d depict illustrative graphs of the average power of an amplitude-modulated signal as a function of time for the three reflected signals, signal 107-2 through signal 107-4, respectively, as they arrive at base station 103-1 after signal 107-1. Typically, signals 107-2 through 107-4 are phase-shifted with respect to signal 107-1, because they each travel a longer path than signal 107-1, and are attenuated to varying degrees, with respect to signal 107-1. Although only four signals are depicted in FIG. 2 as reaching base station 103-1, in practice, many signals typically reach base station 103-1, each having traveled a different path, such that they interfere to form a composite signal at base station 103-1. This phenomenon is known as the xe2x80x9cmultipathxe2x80x9d problem. To simplify the illustration, the relative phase-shift of the constituent signals has been confined to an integral number of wavelengths of the carrier frequency. The resulting composite signal is shown in FIG. 3e. 
In typical analog wireless systems in the prior art, the presence of secondary reflected signals at the receiver interfere with the direct path signal. When the system carries television signals, the reflected multipath signals can appear as xe2x80x9cghostsxe2x80x9d on the screen of older television sets.
In a code-division multiple access (xe2x80x9cCDMAxe2x80x9d) wireless telecommunications system each radio receiver endeavors to identify and isolate the highest-quality constituent signals incident on the receiver and to demodulate and combine them to estimate the transmitted signal. As is well-known in the prior art, this process is conducted with, among other things, a finger-assignor and a rake receiver. The finger-assignor analyzes the incoming composite signal, in well-known fashion, and attempts to identify the strongest constituent signals in the composite signal to the rake receiver. The rake receiver isolates and demodulates each of the identified strongest constituent signals, and then combines the demodulation result from each constituent signal, in well-known fashion, to produce a better estimate of the transmitted signal than could be obtained from any single constituent signal. To accomplish this, a rake receiver comprises a plurality, but finite number, of individual receivers, known as xe2x80x9cfingers,xe2x80x9d each of which isolates and demodulates one constituent signal.
As the wireless terminal moves, the relative signal quality and phase-shift of the constituent signals changes, sometimes considerably. Received constituent signals can disappear, new constituent signals can appear, and existing constituent signals can merge or diverge. The signal quality of a constituent signal can suffer radical momentary changes, which make it appear for a time that the constituent signal no longer exists, although it quickly reappears. Such changes can be due to, for example, Rayleigh fading, or the transmitter passing behind an obstruction. Furthermore, the current receiver techniques in some CDMA technologies (e.g., IS-95 CDMA Rate Set 1) are such that usable constituent signals can have signal qualities so close to that of the noise floor that often random fluctuations in the noise floor can appear to the finger-assignor to be genuine signal constituents. These are called spurious signals, and, when assigned to the rake receiver: (1) degrade the quality of the demodulator""s output, and (2) take up a finger in the receiver which could otherwise be profitably assigned to a genuine constituent signal.
Because it is well-known to those skilled in the art that the inclusion of spurious signals in the combination process is detrimental to the operation of the system, a technique has been developed for addressing the issue. In accordance with this technique, constituent signals are identified and assigned to available fingers in well-known fashion. However, a newly assigned constituent signal is initially put on probationxe2x80x94meaning that the newly assigned constituent signal is not included in the combination process until the signal has maintained a measure of signal quality above a threshold, C, for a predetermined amount of time. Thereafter, the signal is included in the combination process. The probation period is advantageous in that it is statistically improbable for a spurious signal to meet the signal quality threshold for the predetermined amount of time and thus it is statistically improbable that a spurious signal will be included in the combination process.
FIG. 4 depicts a graph of the signal quality as a function of time of a constituent signal that has been assigned to a finger at t0. Because the signal maintains a signal quality at all times above the threshold, C, until time tp, the signal is included in the combination process after time tp.
A constituent signal may be de-assigned for various reasons. One reason a constituent signal may be de-assigned is that it is to be replaced with a constituent signal of apparently higher quality. Another reason is that the de-assigned constituent signal may have been determined to have become spurious. According to prior art, the constituent signal in FIG. 4 is de-assigned for this latter reason because its signal quality fell below the rejection threshold, R, for time tr.
Despite its advantages, the prior art has several disadvantages. First, the prior art does not allow the utilization of genuine signals in the combination process until after the probation period, which deprives the combination process of valuable information. This disadvantage could be ameliorated by shortening the probation period, but shortening the probation period would admit a significant number of spurious signals into the combination process. Furthermore, by shortening the probation period, the de-assignment process is compromised in attempting to satisfy the conflicting goals of de-assigning spurious signals quickly, while retaining genuine signals through momentary periods of degraded signal quality.
Therefore, the need exists for an improved technique for de-assigning signals from the fingers of a rake receiver.
Some embodiments of the present invention are capable of de-assigning signals from the fingers of a rake receiver without some of the costs and disadvantages of techniques in the prior art. In particular, some embodiments of the present invention de-assign spurious signals from the fingers of a rake receiver quickly and do not de-assign genuine signals in momentary periods of degraded signal quality.
Some embodiments of the present invention are advantageous because they enable the rake receiver to, on average, have a larger number of genuine constituent signals and a smaller number of spurious signals included in the combination process, which results in a higher-quality estimate of the transmitted signal. Furthermore, some embodiments of the present invention enable the inclusion of newly-assigned constituent signals in the combination process without a probation period.
Because embodiments of the present invention can substantially affect the traffic capacity of a wireless telecommunications system, it will be clear to those skilled in the art that embodiments of the present invention can affect the cost-effectiveness of an entire wireless telecommunications system.
In general, embodiments of the present invention accomplish this goal by: (1) placing more stringent standards on newly assigned signals, which may be spurious, and (2) placing less stringent standards on mature signals that have proved themselves over time but may experience only a temporary signal quality shortfall.
Illustrative embodiments of the present invention accomplish this goal by using one or more of the following four techniques.
The first technique de-assigns a signal from a finger when a measure of signal quality of the signal crosses a threshold, while changing the threshold as a function of the duration that the signal has been assigned to the finger.
The second technique de-assigns the signal from the finger when a time-average measure of signal quality of the signal crosses a threshold, while changing the length of time in which the time-average measure of signal quality is determined as a function of the duration that the signal has been assigned to the finger.
The third technique de-assigns the signal from the finger when a measure of signal quality of the signal spends more than a percentage of time below a threshold, while changing the percentage as a function of the duration that the signal has been assigned to the finger.
The fourth technique de-assigns the signal from the finger when a measure of signal quality of the signal spends more than a percentage of time below a threshold during an interval of time of a first length, while changing the first length as a function of the duration that the signal has been assigned to the finger.